[TowerTalk] W0IYH Feed line Choke Performance

At 06:17 AM 8/20/03 -0700, Jim Lux wrote:
>Let's consider why you're doing all this choking in the first place:
>a) to reduce the perturbations from feedlines on the antenna pattern. In
>this case, breaking the line up into short isolated segments (like guy
>wires) that are sufficiently "out of band" should work nicely.
>b) to reduce imbalance. Choking at the feed point works here, provided you
>have already done a) so that you don't get common mode induced currents
>coming in from the near field.
In receiving applications, there is also the sub-set of concerns due to
pickup on the shield of the coax destroying the directionality of antennas
like pennants, flags, etc. ON4UN has an interesting device in his book on
low-band DXing (see page 7-19), involving two bead chokes with a ground
between them, placed at least 5 meters from the feedpoint of the receiving
antenna. He claims this offers a very substantial attenuation of common
mode currents (approximately 70 dB).
73, Pete N4ZR
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